1. Field of the Invention
This invention relates to a developing unit and a developer stirring and transporting method.
2. Description of the Related Art
Conventionally, in discharging a developer in a developing unit from the developing unit, the developer is discharged by overflow from a developer discharge port formed in a sidewall of the developing unit. In this structure, in order to cause overflow more easily, a technique is known in which a rotary blade near the developer discharge port of a second transport member that stirs and transports the developer is replaced by a rotary blade having less transporting power than an ordinary one, thereby locally raising the height of the top surface of the developer near the developer discharge port to a position higher than the other areas (see, for example, JP-A-2000-81787).
In such a developing system, a carrier is supplied together with toner into the developing unit by a small amount each, and excessive developer is caused to overflow from the developer discharge port, thus constantly replacing degraded developer. Therefore, development performance can be maintained and deterioration in image quality can be restrained. Since batched replacement of the developer is not carried out, good maintenance property can be kept. Also, since a small quantity of developer suffices, the developing unit can be miniaturized and the machine can be miniaturized. However, as the machine is miniaturized, it can be put in various places such as on the floor or desk, and it is less often installed in a horizontal place. In the foregoing developing system, since the developer is discharged by overflow, the height position of the top surface of the developer with respect to the discharge port of the developing unit varies depending on the degree of inclination of the machine, and this largely affects the discharge of the developer.
For example, the developer is discharged satisfactorily in a horizontal state, but if the machine is inclined when it is installed, the developing unit is inclined and the height position of the top surface of the developer with respect to the discharge port is lowered. Thus, the developer will not be discharged. As the height position of the top surface of the developer with respect to the discharge port is lowered, the developer will not be discharged unless its volume increases unnecessarily. Therefore, the quantity of the incoming developer into the developing unit increases largely. As a result, the developer may spill, or excessive supply to the developing roller or change in the proportion of toner and carrier occurs, causing uneven density. Thus, a problem arises that high image quality cannot be maintained.